Chemical vapour deposition ("CVD") of alumina on cutting tools is a well established technology. The wear properties of Al.sub.2 O.sub.3 as well as of other refractory materials have been discussed extensively in the literature.
The CVD-technique has also been used to produce coatings of other metal oxides, carbides and nitrides, the metal being selected from transition metals of the IVB, VB and VIB groups of the Periodic Table of the Elements. Many of these compounds have found practical applications as wear resistant or protective coatings, but few have received as much attention as TiC, TiN and Al.sub.2 O.sub.3.
Al.sub.2 O.sub.3 crystallizes in several different phases: .alpha., .kappa., .gamma., .beta., .theta., etc. The two most frequently occurring phases in CVD of wear resistant Al.sub.2 O.sub.3 -coatings are the thermodynamically stable, hexagonal .alpha.-phase and the metastable .kappa.-phase. Generally, the .kappa.-phase is fine-grained with a grain size in the range 0.5-3.5 .mu.m and often exhibiting a columnar coating morphology. Furthermore, .kappa.-Al.sub.2 O.sub.3 coatings are free from crystallographic defects and free from micropores or voids. The crystallographic structure of .kappa.-Al.sub.2 O.sub.3 is basically orthorombic and the unit cell can be expressed with orthogonal indices which might be transformed to a hexagonal unit cell with hexagonal indices.
The .alpha.-Al.sub.2 O.sub.3 grains in mixed .alpha.+.kappa.-Al.sub.2 O.sub.3 coatings are usually coarser with a grain size of 1-6 .mu.m depending upon the deposition conditions. Porosity and crystallographic defects are in this case frequently occurring.
On commercial cutting tools, Al.sub.2 O.sub.3 is always applied on TiC.sub.x N.sub.y O.sub.z coated carbide or ceramic substrates (see, e.g., U.S. Pat. No. 3,837,896, now Reissue U.S. Pat. No. 29,420 and U.S. Pat. No. 5,071,696) and the interfacial chemical reactions between the TiC.sub.x N.sub.y O.sub.z -surface and the alumina coating are of particular importance.
The practice of coating cemented carbide cutting tools with oxides to further increase their wear resistance is in itself well known as is evidenced in, e.g., U.S. Pat. Reissue No. 29,420 and U.S. Pat. Nos. 4,399,168, 4,018,631, 4,490,191 and 4,463,033. These patents disclose oxide coated bodies and how different pretreatments, e.g., of TiC-coated cemented carbide, enhance the adherence of the subsequently deposited oxide layer. Alumina coated bodies are further disclosed in U.S. Pat. Nos. 3,736,107, 5,071,696 and 5,137,774 wherein the Al.sub.2 O.sub.3 -layers comprise .alpha., .kappa. and/or .alpha.+.kappa.-combinations.
U.S. Pat. No. 4,619,866 describes a method for producing fast growing Al.sub.2 O.sub.3 -layers by utilizing a hydrolysis reaction of a metal halide under the influence of a dopant, e.g., hydrogen sulphide (H.sub.2 S). The resulting coating consists of a mixture of the smaller .kappa.-grains and the larger .alpha.-grains. The process yields coatings with an even layer thickness distribution around the coated body.
U.S. Pat. No. 5,071,696 discloses a body coated with single-phase .kappa.-Al.sub.2 O.sub.3 deposited epitaxially onto an MX-layer where MX is a carbide, carbonitride or oxycarbonitride of a metal from the Groups IVB, VB or VIB of the periodic table. Swedish Patent Application No. 9101953-9 discloses a method of growing a fine-grained .kappa.-alumina coating.
Single-phase .alpha.-Al.sub.2 O.sub.3 -layer with preferred crystal growth orientation in the (012)-direction is disclosed in Swedish Patent Application No. 9203852-0, in the (110)-direction is disclosed in Swedish Patent Application No. 9304283-6 and in the (104)-direction is disclosed in Swedish Patent Application No. 9400089-0.